1. Field of the Invention
The present invention relates to a data processor, and more particularly to a data processor which comprises a semiconductor memory device that requires a precharge when data is read therefrom.
2. Description of the Related Art
A data processor such as a microprocessor operates in synchronization with a clock within a predetermined frequency range, and is therefore prone to malfunctions if the clock greatly deviates in frequency. To address this problem, Japanese Patent Laid-Open No. 55130/2002, for example, discloses a data processor which comprises a frequency determination circuit for determining whether or not the clock falls within a predetermined range of frequency, for resetting the operation of a CPU when the clock deviates in frequency to prevent malfunctions.
The data processor described in Japanese Patent Laid-Open No. 55130/2002 comprises a frequency detector circuit composed of capacitor C and resistor R. Capacitor C repeats charging and discharging at each timing at which an externally applied clock varies in frequency. The frequency detector circuit delivers an error detection signal when the charging or discharging is not completed within one period of the clock (when the clock frequency is high).
In the conventional data processor as described above, since the frequency detector circuit is composed of capacitor C and resistor R, it encounters difficulties in precisely determining whether or not the clock frequency exceeds a predetermined frequency due to a change in detection characteristics caused by a varying power supply voltage and ambient temperature, variations related to the manufacturing, and the like. Another problem implied in the conventional data processor is a change in the detection characteristics of the frequency detector circuit depending on use conditions after shipment.
In addition, the use of capacitor C and resistor R results in a large area required for the layout of the frequency detector circuit, disadvantageously leading to an increased chip area, for example, when the data processor is integrated in one chip.